Light collecting systems such as wafer inspection systems can use area illumination to illuminate areas of a sample. Light reflected and/or scattered from an illuminated area is collected by collection optics and detected by detectors. Light collecting systems can alternatively use a scanning light beam that scans the sample.
The intensity (and possibly other characteristics) of the collected light is compared to an expected range of values. Deviations of the scattered light from the expected range can be indicative of defects in or on the substrate.
The sensitivity of a light collecting system depends on its signal/noise ratio (SNR). In this context, the signal corresponds to the amount of light scattered from the defect that is able to reach the detector. The “noise” is generally dominated by background light that is reflected or otherwise scattered from the substrate itself. (In the context of the present patent application and in the claims, the term “scattered” refers to all radiation returned from the surface, due to substantially any physical mechanism, including diffractive scattering and both specular and diffuse reflection.)
When a patterned semiconductor substrate is illuminated with coherent light, for example, the light is diffracted from the repetitive pattern and generates constructive interference lobes along well-defined directions. The positions and extent of the interference lobes depend on the period of the pattern, as well as the wavelength of the incident radiation and characteristics of the optical system. Accordingly, in order to block different interference patterns a very large number of spatial filter configurations are required.
It is known in the art that blocking the interference lobes can facilitate the detection of defects and pattern irregularities on the substrate. For example, U.S. Pat. No. 3,614,232, to Mathisen, whose disclosure is incorporated herein by reference, describes a spatial filter for detecting defects in photomasks, using a transmission geometry and a simple filter consisting substantially of the negative of the Fourier transform of a defect-free specimen of the microcircuit.
Filtering ultra violet light as well as deep ultra violet light and even extreme deep ultra violet light is very problematic as many spatial filters (including for example liquid crystal spatial filters) are opaque to this radiation.
There is a need to provide efficient spatial filters and efficient systems.